1. Field of the Invention
The present invention relates to a flat panel display device and, more particularly, an active matrix type liquid crystal display device.
2. Description of the Related Art
A flat panel display device which is typified by a liquid crystal display device is employed in various fields to profitably employ characteristics of light weight, small thickness, and low power consumption. In particular, the liquid crystal display device in which a liquid crystal layer is employed as an optical modulation layer is widely employed as a display device for OA devices, home electrical appliances, etc. Especially an active matrix type liquid crystal display device in which switch elements are provided to pixels one by one is spread rapidly as the display device of the OA device.
In the liquid crystal display device, when the voltage of the video signal applied to the liquid crystal layer is unipolar, a direct current (DC) component is always applied to the liquid crystal layer. Problems such as deterioration of the liquid crystal layer are caused if such condition is continued for a long time. In order to prevent such problem, the polarity inversion driving method is executed, i.e., polarity of the voltage applied to the liquid crystal layer is inverted every frame period.
Meanwhile, a video signal is applied to the liquid crystal panel at a timing which is in synchronism with a reference clock signal being input from an external device. For this reason, if the reference clock signal is in a non-conformity state due to any reason, the DC component is applied to the liquid crystal layer over a long time. This situation causes deterioration of the liquid crystal and in turn causes abnormality in image display.
Therefore, such a technology has been proposed that a clock monitoring circuit is provided in the liquid crystal display device and thus the DC component can be prevented from being applied to the liquid crystal layer over a long time when the reference clock signal is in a non-conformity state . However, according to such configuration in the prior art, there are problems such that the number of parts to be incorporated is increased and it is difficult to use the parts in common. As a result, it is difficult to achieve improvement in productivity and a lower cost of the device.